As a method replacing a conventional radiography using a combination of a radiographic film and radiographic intensifying screen, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has been practically employed. This recording and reproducing method employs a radiation image storage panel (which is also referred to as "stimulable phosphor sheet") comprising a support and a stimulable phosphor layer provided thereon, and the procedure of the recording and reproducing method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light for obtaining electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The panel thus treated is then subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In general, the above radiation image storage panel (i.e., stimulable phosphor sheet) has a basic structure comprising a support, a stimulable phosphor layer, and a protective film overlaid in order. The stimulable phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor with no binder. The phosphor layer containing no binder can be formed by a deposition process or a firing process. Further, a phosphor layer comprising an agglomerated phosphor soaked with a polymer is also known. For the recording and reproducing method, the radiation image storage panels having any types of stimulable phosphor layer are employable.
The radiation image recording and reproducing method is often used in X-ray radiography for medical diagnosis. In that case, it is especially desired to obtain a radiation image of high quality (particularly, high sharpness for high resolution) by applying a relatively small dose of radiation. Therefore, the radiation image storage panel is required to have a high sensitivity and also to provide an image of high quality.
The sharpness of radiation image is mainly affected by diffusion of the stimulating rays in the phosphor layer. In the reproducing process, the procedure for reading the latent image is performed by the steps of sequentially and sweepingly applying a beam of the stimulating rays onto the surface of the phosphor layer to induce the stimulated emission and successively collecting the emission. If the stimulating rays diffuse (horizontally, in particular) in the phosphor layer, they excite the phosphor particles not only at the target spot but also in its periphery area. Consequently, the stimulated emission emitted from the target spot is collected in combination with stimulated emissions emitted from the periphery area. The contamination of the target emission with the emissions from the periphery impairs the sharpness of the resultant radiation image.
For avoiding the diffusion of the stimulating rays, it was proposed to divide the plane of the stimulable phosphor layer into small sections (cells) with a partition which reflects the applied stimulating rays.
Japanese Patent Provisional Publication No. 59-202100 discloses a radiation image storage panel having a honey-comb structure consisting of many small cells filled with stimulable phosphor. The storage panel comprises a substrate and a stimulable phosphor layer provided thereon, and the honey-comb structure sectioned with a partition is further provided on the phosphor layer.
Japanese Patent Provisional Publication No. 62-36599 discloses a storage panel employing a support provided with many hollows regularly arranged on one surface. The hollows are filled with stimulable phosphor, and the ratio of depth to diameter of each hollow is 3.5 or more.
Japanese Patent Provisional Publication No. H5-512636 discloses a process for preparing pixel phosphor with a mold.
Japanese Patent Provisional Publication No. H2-129600 discloses a radiation image storage panel employing a support plate having many holes vertically bored and filled with stimulable phosphor.
Further, Japanese Patent Provisional Publication No. H2-280100 discloses a stimulable phosphor sheet employing a substrate having a honey-comb micro-structure filled with stimulable phosphor.
Japanese Patent Provisional Publication No. H2-176600 describes a stimulable phosphor sheet having a phosphor layer comprising a stimulable phosphor and a phosphor that is excitable by radiation to emit a light in the ultraviolet region, in which the stimulable phosphor absorbs and stores thus emitted ultraviolet light. The sensitivity of the stimulable phosphor panel is increased by the combination of UV light-emitting phosphor and UV light-absorbing stimulable phosphor, because a portion of the applied radiation which is not directly absorbed by the stimulable phosphor but is absorbed by the UV light-emitting phosphor increases the radiation absorption.
In the known radiation image storage panel employing a support or substrate provided with many holes or hollows charged with phosphor, a part of support or substrate serves as a partition keeping the simulating rays from diffusion. Thus prepared radiation image storage panel, therefore, gives a radiation image of high quality (particularly, high sharpness). On the other hand, since the partition of support material partly occupies the phosphor layer in the storage panel, the amount of the phosphor charged in a unit volume of the layer is made relatively small for absorbing an enough amount of X-ray radiation. Consequently, the provision of partition lowers the sensitivity of the storage panel. Although the sensitivity can be enhanced by increasing the thickness of the stimulable phosphor layer, a thick phosphor layer generally impairs the sharpness of the reproduced radiation image.
In X-ray radiography for medical diagnosis, a storage panel or a phosphor sheet of high sensitivity reduces a dose of radiation applied to a patient. Therefore, it is desired to provide a radiation image storage panel or a stimulable phosphor sheet giving an image of high sharpness as well as high sensitivity.